As it is well known, demands for circuits with higher operating speeds, affecting at present most electronic applications, particularly at radio frequency, and the need to produce chip integrated mechanical structures (MEMS-Micro Electro-Mechanical System), make the use of SOI (Semiconductor On Insulator) substrates more and more frequent. In fact, most of the limitations of electronic-circuit performance are due to the dispersion and parasitic effects occurring between the integrated devices and the substrate.
A first prior-art solution to insulate the semiconductor integrated electronic devices from the substrate provides for the creation under said devices of highly doped “buried” wells, which form charge-depletion areas in order to block the charge flow towards the substrate.
Although advantageous in many aspects, this first solution has several drawbacks. In fact, with the increase of circuit operating frequencies, this solution cannot meet the insulation requirements needed for the correct operation of the single devices.
As already mentioned, a second solution provides, on the contrary, the use of SOI substrates, whose structure is shown in FIG. 1.
In particular, a SOI structure comprises a first substrate 1 and a second substrate 2 insulated from each other by an oxide layer 3. Such a structure allows the first substrate 1, whereon all devices are integrated, to be electrically insulated from the second substrate 2.
It is evident that the dispersion currents between the first and second substrates are almost null, and the parasitic capacitance effects are reduced, allowing, therefore, the integration of devices operating at higher cut-off frequencies than typically possible in a non-SOI structure.
As already said, SOI structures are used considerably also in MEMS and MOEMS (Micro Optical-Electro-Mechanical System) applications to form suspended structures, because it is possible to selectively remove the buried oxide layer since it is used as sacrificial layer.
Nevertheless, the formation of SOI structures is rather expensive, and it heavily affects the calculation of the final cost of the finished product. Moreover, these manufacturing processes are rather critical and often involve a significant decrease in yield, both because of structural defects and because of the effective reduction in useable area.
The main process steps of a traditional SOI structure manufacturing process are now described.
A first and second substrate 1 and 2 are superficially oxidized, so that the thickness of the oxide layer 3 thus obtained on the surface of the two substrates 1 and 2 is about half the thickness of the final layer 3 as shown in FIG. 2.
After performing a polishing step of the surfaces which will come into contact, the first and second substrates 1 and 2 are initially aligned with micrometric precision, and are then put in surface contact. By means of a pressure exerted by a body B, in order to eliminate the air between the oxide layers 3 belonging to two substrates 1 and 2, a virtually perfect adhesion is obtained as shown in FIG. 3.
The first and second substrates 1 and 2 are then subjected to a thermal process at high temperature (e.g., 1100÷1200° C.) which allows the oxide layer 3 to reflux and then bond of the two substrates (FIG. 4).
The SOI structure thus obtained is superficially refaced and polished in order to obtain the surface ready for the integration of the electronic devices. The thickness of the first substrate is set at a chosen value, which varies according to the applications used as shown in FIG. 5.
Although the process can seem simple, the high cost of the single SOI structure is, however, due to the low yield. In fact, in order to make the SOI structure usable, the alignment of the two substrates 1 and 2 must be practically perfect, with the ideal being to keep the crystallographic orientation of the substrates 1 and 2 the same in the SOI structure.
Moreover the thermal processes create a series of stress phenomena which produce the so-called “warp” effect. This effect causes a bending of the SOI structure because of a bending-radius decrease by more than one order of magnitude. This “warp” effect is even higher if the oxide layer 3 on the back side of the substrate 2 facing away from the substrate 1, of the SOI structure is removed, because this layer balances at least partially the “warp” effect induced on the SOI structure by the oxide layer 3 on the first substrate 1. The removal of this back oxide layer 3 is quite common in most devices having a back contact. In some SOI structures, the bending is so high as to create problems in the photolithography both in the alignment step and in the focusing step. Moreover, in some cases, the “warp” effect of the SOI structure does not allow the vacuum system that holds the SOI structure in place to seal.
It is therefore expensive from the economic point of view to manufacture SOI structures characterized by close tolerances from the point of view of thickness, uniformity, and “warp” effect. Moreover the external circular oxide crown called the “terrace” must be taken into consideration for each SOI structure, because it reduces, by more than 1 cm, the diameter of the area effectively usable for the integration. Also, the terrace negatively affects the wafer yield, greatly on small and middle-sized substrates (4, 5 and 6 inches), and in a more limited way on large-sized substrates (8 and 12 inches).
A second method to form higher-quality substrates, but also more expensive, exploits the so-called SIMOX method, whose main steps are indicated herebelow.
With particular reference to FIGS. 6 to 9, a semiconductor substrate 4 is shown, which is implanted with high-energy and high-dose oxygen ions. The implanted layer 5 which is formed is positioned at about 1 μm from the surface.
An annealing is then performed at extremely high temperature in an inert atmosphere for the diffusion of the implanted ions and the restoration of the crystalline characteristics of the semiconductor substrate above the layer 5 damaged by ion implantation, with subsequent extension of the area which will become the SOI structure oxide layer.
A further annealing step at extremely high temperature in an oxidizing atmosphere is performed to realize the oxidation of the buried layer 5 and complete the SOI substrate structure oxide layer 5′.
The surface oxide layer 6 is then removed to complete the definite SOI structure.
Although the SIMOX method allows improved substrates to be obtained, from the physical-mechanical characteristic point of view, this process is, as already mentioned, rather expensive. Moreover, limitations exist on the achievable thickness; for example, the thickness of the buried oxide layer typically does not exceed 500 nm, and the thickness of the overall SOI structure typically does not exceed one micron.
If compared to the first method for manufacturing a SOI structure, this last method allows an improved thickness tolerance, a greater uniformity, the absence of the terrace and, thus, a larger exposable surface for the same wafer size, and finally the so-formed SOI structures are less affected by the “warp” effect.
Therefore, a need has arisen for a structure comprising a low-cost buried insulation layer capable of providing good physical-mechanical characteristics and a wide thickness choice of the buried oxide layer and of the final structure, overcoming the limitations and drawbacks which still limit the SOI structures formed according to the prior art.